U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY Terranes and Overlap Sequences in the Central and Southern Appalachians, An Expanded Explanation for Part of the Circum-Atlantic Terrane Map by J. Wright Horton, Jr.1, Avery Ala Drake, Jr.1, and Douglas W. Rankin1 Open-File Report 94-682 Prepared in cooperation with International Geological Correlation Program Project 233, "Terranes in the Circum-Atlantic Paleozoic Orogens." This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. ^Reston, Virginia 1994 CONTENTS Foreword ii Introduction 1 Pre-Mesozoic overstep sequences 1 m-10: Possible Middle to Late Ordovician overstep sequence 1 Terranes 1 NAld: Westminster terrane 1 NAle: Hamburg terrane 2 NA2e: Jefferson terrane 3 NA2g: Potomac composite terrane 5 NA2h: Smith River terrane 6 NA2i: Inner Piedmont composite terrane 7 NA28: Baltimore terrane 8 NA29: Sauratown terrane 9 NA30: Pine Mountain terrane 10 NA31: Gaffney terrane 11 NA32: Bel Air-Rising Sun terrane 11 NA33: Chopawamsic terrane 13 NA34: Milton terrane 14 NA35: Uchee terrane 15 NA36: Juliette terrane 16 NA37a: Carolina terrane 16 NA37b: Spring Hope terrane 18 NA37c: Roanoke Rapids terrane 19 NA37d: Charleston terrane 20 NA38: Falls Lake terrane 21 NA39: Crabtree terrane 21 NA40: Goochland terrane 22 NA41: Wilmington terrane 23 NA42: Sussex terrane 24 NA43: Hatteras terrane 24 NA44a: Suwannee terrane 25 NA44b: St. Lucie Metamorphic Complex (part of Suwannee terrane) 26 Acknowledgments 27 References Cited 28 FIGURES Figure 1. Accretionary diagram for the central Appalachian region 39 Figure 2. Accretionary diagram for the southern Appalachian region 40 FOREWORD The following text was submitted for publication in 1989 as part of a "terrane catalogue," which was intended to serve as an expanded explanation for the Tectonic Map of Pre-Mesozoic Terranes in Circum-Atlantic Phanerozoic Orogens (Keppie and Dallmeyer, 1989) in cooperation with International Geological Correlation Program Project 233. Plans to publish the terrane catalogue have been cancelled, because texts for some of the other regions were not completed. Consequently, this open-file report is being released in order to make the expanded explanation for the central and southern Appalachian part of the circum-Atlantic terrane map available. Although some of the material is outdated or superseded, it is important for this report to agree with the published map and to reflect the state of knowledge and thinking at the time it was produced. Therefore, the text is presented as it was written in 1989 except for updating of references that were "in press" at the time and for the addition of references cited in this "foreword." Numerical designations for terranes match those on the published map (Horton and others, 1989b; Keppie and Dallmeyer, 1989). Terrane concepts in eastern North America have evolved since the circum-Atlantic terrane map was published. Interpretations of tectonostratigraphic terranes in the central and southern Appalachians based on more current information are available in Horton and others (1991), Goldsmith and Secor (1993), Rankin and others (1993a, 1993b), and Rankin (1994). INTRODUCTION This section of the terrane catalogue to accompany the Tectonic Map of Pre-Mesozoic Terranes in Circum-Atlantic Phanerozoic Orogens (Keppie and Dallmeyer, 1989), covers the central and southern Appalachian region of the United States, from New York City to central Florida. Alphanumeric symbols for terranes and an overstep sequence are keyed to the map. The circum-Atlantic terrane map of this region (Horton and others, 1989b) and the following descriptions of tectonostratigraphic terranes and overlap sequences are adapted from Horton and others (1989a). Readers interested in more detailed discussions of these terranes and their Paleozoic histories of amalgamation and accretion should consult that report. Accretionary-history diagrams for the central Appalachians (Fig. 1) and southern Appalachians (Fig. 2) are modified from those on the terrane map (Horton and others, 1989b; Keppie and Dallmeyer, 1989). PRE-MESOZOIC OVERSTEP SEQUENCES m-10: POSSIBLE MIDDLE TO LATE ORDOVICIAN OVERSTEP SEQUENCE The Chopawamsic Formation of the Chopawamsic terrane is unconformably overlain by the paleontologically dated Middle and Upper Ordovician Arvonia Slate and Quantico Formation (Pavlides, 1980). Because the Arvonia and Quantico are post-Taconian, the Chopawamsic terrane is overlapped by them. The Arvonia Slate contains assemblages of fossil brachiopods, trilobites, bryozoans, and echinoderms, which collectively indicate an open marine environment (Kolata and Pavlides, 1986, and references therein). The trilobites indicate a Middle to Late Ordovician age (Tillman, 1970). The similar and probably correlative Quantico Formation has yielded segments of crinoid stems and a few other fossils, and is also probably Late Ordovician (Pavlides and others, 1980). The Peach Bottom Slate in Maryland may correlate with the Arvonia Slate and Quantico Formation in Virginia (Drake and others 1989); if so, all of these would collectively constitute a late Taconian or post-Taconian overstep sequence across the Potomac and Chopawamsic terranes (Horton and others, 1989a). The overstep interpretation is illustrated diagrammatically on Figure 1. In addition, mapping by Gates and others (1986) suggests that the southwestern end of the Arvonia Slate may overstep the boundary between the Chopawamsic and Milton terranes. REFERENCES-Gates and others (1986); Horton and others (1989a); Kolata and Pavlides (1986); Pavlides (1980);Pavlides and others (1980); Tillman (1970) TERRANES NAld: WESTMINSTER TERRANE The Westminster terrane of Muller and others (1989) and Horton and others (1989a) was previously termed the Ijamsville-Pretty Boy-Octoraro terrane by Horton and Drake (1986). TERRANE BOUNDARIES~The northwest boundary of this terrane is the Martic overthrust, which brings it onto the Honey Brook Upland and the Blue Ridge tectonic province. Its southeast boundary, with the Potomac composite terrane, is the Pleasant Grove-Huntingdon Valley fault, which is interpreted to be the Taconian suture. STRATIGRAPHY--The Westminster terrane (Fig. 1) consists of a unit of pelitic schist or phyllite characterized by albite porphyroblasts (Prettyboy Schist of Crowley, 1976, and Octoraro Phyllite) and a green and purple phyllite unit (Ijamsville Phyllite) that is sodium-rich and characterized by the mineral assemblage muscovite-paragonite-chloritoid. The terrane also contains metabasalt of the Sams Creek Formation, which has not yet been chemically characterized, as well as Wakefield Marble and Silver Run Limestone. DEFORMATIONAL AND METAMORPHIC EVENTS-Theterrane-boundary thrust faults date from the Taconian orogeny. Both of these major faults later experienced dextral strike-slip motion during the Alleghanian orogeny. The rocks of the terrane were interpreted to be polymetamorphic by Stose and Stose (1946), although this interpretation has yet to be confirmed by modern work. IGNEOUS INTRUSIONS-None have been reported. EVIDENCE FOR TERRANE IDENTITY AND TIME OF AMALGAMATION-Rocks of the Westminster terrane are interpreted to be part of the rise-prism deposited off the Laurentian continental margin and to correlate with rocks in the Hamburg terrane and some of the higher slices of the Taconic allochthon in New England and New York (Knopf, 1935; Lyttle, 1982; Drake, 1986; Drake and others, 1989). REFERENCES-Crowley (1976); Drake (1986); Drake and others (1989); Horton and Drake (1986); Horton and others (1989a); Knopf (1935); Lyttle (1982); Muller and others (1989); Stose and Stose (1946) NAle: HAMBURG TERRANE TERRANE BOUNDARIES~The Hamburg terrane, originally named by Williams and Hatcher (1982), occurs in several allochthonous slices that include the Hamburg, Jutland, Peapack, Cocalico, and Furlong klippen (Lyttle and Epstein, 1987; Drake and others, 1989). Slices of the Hamburg terrane occur only on Laurentian cover rocks of the Reading Prong and Honey Brook Upland. STRATIGRAPHY-Much of the Hamburg terrane consists of variegated shale and mudstone, deep-water limestone, chert, some basalt and mafic intrusive rocks of Early Ordovician age, and turbidites and related shales of Middle Ordovician age (Fig. 1). These rocks represent abyssal deposits and younger submarine fan deposits. The Hamburg klippe also contains a slice of siliciclastic and carbonate rocks of Late Cambrian and Early Ordovician age that was deposited at the base of a slope. The Furlong klippe consists of variegated shale, quartzite, and conglomerate interpreted as a rift sequence of Late Proterozoic to earliest Cambrian age. DEFORMATIONAL AND METAMORPHIC EVENTS~The terrane was probably assembled with rock of the Reading Prong and Honey Brook Upland by thrusting during Taconian foreland basin sedimentation in the late Middle Ordovician, but most contacts seen now result from later thrust faulting, probably during both the Taconian and Alleghanian orogenies. IGNEOUS INTRUSIONS Minor amounts of mafic intrusive rocks occur with pillow lavas and flow breccias in the Hamburg klippe. These rocks have both calc-alkaline and oceanic basalt affinities and are interpreted to result from near-trench volcanism (Lash, 1986). EVIDENCE FOR TERRANE IDENTITY AND TIME OF AMALGAMATION-The